Selenium rectifiers



Patented Aug. 3, 1948 SELENIUM RECTIFIERS Stanley S. Fry, North Chicago,Ill., assignor to Fansteel Metallurgical Corporation, North Chicago,Ill.. a corporation of New York No Drawing. Application June 14, 1945,

Serial No. 599,511!

13 Claims. (Cl. 175-366) This invention relates to selenium rectifiers.

.In the production of selenium rectiflers, a rigid carrier electrode orsupporting electrode is provided with a thin coating or layer ofselenium. The supporting electrode may be formed of nickel,nickel-plated iron, aluminum, magnesium, beryllium or other metals andalloys. A common practice in the production of selenium rectifiersincludes grit blasting the iron disc or plate and electroplating thedisc with nickel, The purpose of the grit blasting is to present aroughened surface to the selenium and to thereby improve the adherenceof the selenium to the carrier electrode. The disc is then thoroughlycleaned and a thin film of selenium is applied over the nickel layer.

The selenium film or layer may be formed by a variety of methods. Theplate or disc may be heated to a temperature above the melting point .ofselenium, for example, to a temperature of from 230 C. to 250 C., andtheselenium in stick form may be rubbed across the heated plate in order tomelt the selenium and formthe desired film. Another method ofapplication includes over the heated'plate by. mechanical means, as witha-glass rod. The selenium may also be deposited on the carrier electrodefrom a. vapor phase. The vaporization method is commonly employed indepositing the selenium filmupon the" light metal carrier electrodes.Various materials may be added to the selenium to increase itsconductivity and otherwise impart --desired characteristics andproperties.

The selenium is then transformed into its gray, crystalline state byheat treatment. The coated discs are generally stacked with mica,aluminum or other inert, smooth-surfaced discs or washers in contactwith the selenium and between adjacent plates, and the stack subjectedto a moderate pressure. The stacks under pressure are then subjected toa relatively low temperature, that is, a temperature below 150 C., andmaintained at such temperature for from one hour to four or five hours.During this stage of the heat treatment the selenium softens so that thepressure produces a layer of selenium oi relatively uniform thicknessand having a smooth surface.

The stacks are withdrawn from the oven or heat treating furnace and theplates are removed from the stack. The plates are then given a furtherheat treatment at a temperature approachm the melting point of selenium.This heat treatment is generally conducted at temperatures between about200 C. and the melting point of selenium, for example, about 210 C., fora period of from fifteen minutes to several hours. During the combinedheat treatment, a layer of selenium is produced having a smooth surfaceof more or less uniform thickness and the selenium is transformed fromits amorphous, non-conducting form into its gray, crystalline,conducting form.

The smooth surface of the selenium film is then treated to form anartificial blocking layer and a counter-electrode consisting of arelatively low melting point alloy is applied, as by spraying, over theselenium coating.

The final step in the manufacture of the rectifier plates consists of anelectrical forming treatment. This treatment consists of subjecting theplate to either an alternating or direct current voltage below about 15volts or a pulsating direct current starting with a low voltage of about8 .volts and gradually increasing the voltage to about 21 volts over aperiod of several hours.

In the use of an alternating current, it is necessary to include in thecircuit a current limiting 'resistor because one-half cyclefiows in theforward direction of the rectifier plate. The voltage may be about 20volts and the period required for forming is generally greater than thatrequire when a direct current is employed because only one-half cycle ofthe current flows in the reverse direction.

Light sensitive devices of the blocking layer class are produced in asimilar manner. A supporting electrode is provided with a thin film orlayer of selenium in its. gray, crystalline modification and anartificial blocking layer formed on the surface of the selenium, Asecond electrode is then applied over the treated surface of theselenium, This electrode may consist of a lightpermeable film of metalor a metal grid. The application of the light-permeable metal may beaccomplished by any of the methods known to the art.

This invention is directed to the manufacture of selenium rectifiersprovided with a specific type of counter-electrode. The method is notlimited to any specific form, type or composition of carrier electrode.Any method may be employed in providing the carrier electrode with thefilm or layer of gray, crystalline, conducting selenium. The seleniumlayer may or may not contain added a selenium rectifier element havinganappreciably higher break down voltage-than selenium rectifiers made withthe usual and conventional low melting point counter-electrode.

Other objects and-advantages of In conventional-methods of production ofselenium rectifier elements, thecounter-electrode consists oi a low.melting point alloy oi bismuth,

cadmium and tin. The particular composition or the alloy differs withvarious manufacturers depending in mostinstances upon individualpreference. Commonly. thealloy consists of 4 5% to 55% bismuth. 'to. 30%tin and 20 to cadmium,- a common alloy containing 54% bismuth,26% tinand 20% cadmium. This alloy has a melting point oi about'l30 C. Anothercommon alloy consists of Wood's metal, contains approximately.50%bismuth, 25% lead, 12.5%

' cadmium and 12.5% tin and has a melting point of about 65.5 C. Varioustin-cadmium alloys and bismuth-cadmium alloys have been proposed. Forexample, in Patent No. 2,235,051

there is disclosed tin-cadmium alloys containing 25% to60% cadmium.-' Analloy of 68% tin and 32% cadmium has a melting point of about 180 C. Thecounter-electrode disclosed in Patentf-No. 2,193,610 consists of 75% tinand 25% cadmium ,and has a melting point of about 165 C.

' Selenium rectifiers provided with counterelectrodes of these alloysare generally operated at an applied alternating current voltage ofabout lito 18 volts since the rectifier elements break down at voltagesin excess of about 18 volts when applied continuously. In some instancesthis voltage limitation has been a disadvantage in the use of seleniumrectifiers.

I have discovered that the addition of small amounts of magnesium to thecounter-electrode alloy greatly increases the voltage which may besafely and continuously applied to the selenium rectifier. The amount ofmagnesium does not appear to be particularly critical so far asexceedingly thin sprayed coating containing magnesium and coating orcovering this thin film with the usual counter-electrode containing nomagnesium so as to provide a better mechanical layer or film.

Selenium rectiflers provided with a magnesium containingcounter-electrode may be operated at applied alternating currentvoltages between and 35 volts without danger of breakdown.

- The production of the selenium rectifiers may be identical to thatcommonly used wherein a this invention will become apparent irom thefollowing description andclaims.

' materials-to impart desired characteristicsi and properties. Anartificial blocking layer may be du -res carrier electrode is providedwith a thin film or layer of selenium in its gray. crystallinemodification. An artificial blocking layer may be pro-' vided by anydesired manner. The counterelectrodeof this invention is applied as byOne of the methods commonly employed in grading and in evaluating thequality of selenium rectifier plates is the measurementof the currentpassed by the plate in the reverse or blocking direction uponapplication of a direct currentvoltage to the plate in the blockingdirectifier plates made in accordance with the present invention and ofrectifier plates made with a conventional counter-electrode, a group ofnickel plated iron discs, each having an area of approximately 3 sq.cm., was provided with a layer-oi selenium in its gray, crystallineform. The discs were heated to a temperature 01' about 240 C. andselenium in stick form rubbed across the plate. The selenium was spreadover the surface of the heated discs with a glass rod. The plates werethen stacked with mica discs covering the selenium layer and the stackplaced under pressure. The stack was heated to a temperature below C.for about 2 hours. pressure was then removed and the plates heated toabout 210? C. and maintained at this temperature for about hour.

The group of plates was then divided into two series. The first seriesof plates was provided with a counter-electrode consisting ofapproximately 54% bismuth. 26% tin and 20% cadmium. The second series oiplates was provided with a counter-electrode consisting of the samealloy to which had been added about 2% magnesium. The discs of theseries of plates provided with the counter-electrode of the inventionwere subjected to an electrical forming treatment by the application ofa rectified alternating current voltage to each plate in the reverse orblocking direction, the rectified voltage reaching a maximum of about 34volts. The discs or plates provided with the conventionalcounter-electrode were subjected to an electrical forming treatment bythe application of a rectified aiternating current voltage to each platein the reverse or blocking direction, the rectified voltage reaching amaximum of about 21 volts. It is not possible to apply a voltage muchgreater than 21 volts to these plates because the counterelectrodes meltand destroy the rectifier upon the application of voltages between about23 volts and 25 volts.

A direct current voltage of 34 volts from a storage battery source wasapplied in the reverse or blocking direction to the individual discs ofeach series. The leakage current or the reverse current flowing in thestandard plates averaged approximately 25 milliamperesl The leakagecurrent in the plates provided with the counterelectrode of thisinvention averaged approximately 2 milliamperes. In the case of thestandard or control plates it was necessary to measure or read thecurrent flow as rapidly as possible because at this applied voltage thecounter-electrode melts in a short period which may be less than oneminute. In the case of the plates having a counter-electrode of this in-The vention, the continuous application of this voltage does not damageor injure the rectifier and such plates may be operated at appliedalternating current voltages up to 34 or 35 volts.

The improvements in the operating characteristics of rectifier platesmade in accordance with this invention are quite apparent from thisdata. It is obvic us that the numerical values of the leakage currentwill differ with different size rectifier plates and also with seleniumhaving various addition agents. The data, however, is representative oithe improvements attained in the practice oi my invention. Theimprovements obtained by the addition of small amounts of magnesium arenot limited to the specific bismuthtin-cadmium alloys set 'iorth in thespecific example but are also obtained when magnesium is added to otherbismuth-tin-cadmium alloys and other low melting point alloys or thetype commonly used in providing selenium rectifier elements withcounter-electrodes.

I claim:

1. A blocking layer device comprising a carrier electrode, a layer ofgray, crystalline selenium and a counter-electrode consistingessentially of hismuth, tin, lead, cadmium and a small amount ofmagnesium, the counter-electrode having a melting point not exceeding180 C.

2. A blocking layer device comprising a carrier electrode, a layer ofgray, crystalline selenium and a counter-electrode consistingessentially 01' bismuth, tin, lead, cadmium and from about 0.25% toabout 2% magnesium, the counter-electrode having a melting point notexceeding 180 C.

3. A selenium rectifier comprising a, carrier electrode, a layer ofgray, crystalline selenium and a counter-electrode consistingessentially of bismuth, tin, lead, cadmium and a small amount ofmagnesium. the counter-electrode having a melting point not exceeding180 C.

4. A selenium rectifier comprising a carrier electrode, a layer of gray,crystalline selenium and a counter-electrode consisting essentially 01bismuth, tin, lead. cadmium and irom about 0.25% to about 2% magnesium,the counter-electrode having a melting point not exceeding 180 C.

5. A selenium rectifier comprising a carrier electrode, a layer 01'gray. crystalline selenium and a counter-electrode consistingessentially oi bismuth, tin, cadmium and a small amount of magnesium.

6. A selenium rectifier comprising a carrier electrode, a layer or gray.crystalline selenium and a counter-electrode consisting essentially ofbismuth, tin, cadmium and from about 0.25% to about 2% magnesium.

7. A selenium rectifier comprising a carrier electrode, a layer oi gray,crystalline selenium and a counter-electrode composed of an alloyconsistcadmium and magnesium in amounts up to about 8. A seleniumrectifier comprising a carrierelectrode, a layer of gray, crystallineselenium and a counter-electrode composed of an alloy consistingessentially of bismuth, cadmium and magnesium in amounts up to about 2%.a

9. A selenium rectifier comprising a carrier electrode, a layer-ot gray,crystalline selenium on the carrier electrode, a thin layer of an alloyof bismuth. tin, cadmium and magnesium on the selenium layer and a layeror an alloy of bismuth,

tin and cadmium on the first named alloy layer.

10. A selenium rectifier comprising a carrier electrode, a layer ofgray, crystalline selenium. a thin layer of'an alloy consistingessentially 01 bis. muth and cadmium and containing magnesium on theselenium layer and an exterior layer of an alloy consisting essentiallyof bismuth and.

cadmium.

11. A selenium rectifier comprising a carrier electrode, a layer orgray, crystalline selenium and a counter-electrode, thecounter-electrode comprising a thin layer of an alloy on the seleniumlayer, the alloy consisting essentially oi bismuth. tin, lead. cadmiumand a small amount of magnesium and having a melting point not exceeding0., and an exterior layer of an alloy over the magnesium containinglayer, the last named alloy consisting essentially of bismuth. tin, leadand cadmium and having a melting point not exceeding 180 C.

12. A selenium rectifier comprising a carrier electrode, a layer ofgray, crystalline selenium and a counter-electrode composed of an alloyconsisting essentially of cadmium, tin and magnesium in amounts up toabout 2%.

13. A selenium rectifier comprising a carrier electrode, a layer ofgray, crystalline selenium, a thin layer or an alloy consistingessentially of cadmium and tin and containing magnesium on the seleniumlayer and an exterior layer of an alloy consisting essentially oi.cadmium and tin.

STANLEY S. FRY.

nnrnnnncss CITED The following references. are of record in the file 01'this patent:

UNITED STATES PATENT S Lorenz Nov. 12, 1940

